Evaluation of precipitated CaCO3 produced from locally available limestone as a reinforcement filler for PVC pipe

The current experimental work aimed at developing PCC through two major process steps: dissolution and precipitation, using raw materials domestically available as SL, which are intensively used in construction inputs. The pH level was the decisive parameter used to determine the time required to complete the dissolution and carbonation processes during precipitation. The optimal pH levels were found to be 13 for dissolution and 7.1 for precipitation, respectively. The produced PCC was characterized based on chemical analysis, crystallinity, and morphology, showing an increment of CaCO3 content exceeding 99%, sharper crystal peaks, and predominantly calcite PCC. The compatibility of the PCC was assessed by incorporating 25%, 50%, 75%, and 100% of PCC with commercial filler, followed by selected mechanical tests, such as stress at yield, density, and elongation at break. The results indicated that mixing ratios of 25%, 50%, and 75% of PCC with the commercial filler met the standards, with stress at a yield above 45 MPa and density within the range of 1.35 to 1.46 g/cm3. However, complete substitution slightly lowered these properties. Nevertheless, the elongation at break was acceptable at all treatment levels.


Dissolution-precipitation (DP)
In this experiment, the carbonation method was selected to produce PCC by following the experimental process diagram shown in Fig. 1.Pre-calcined SL was obtained from the Derba cement factory, along with 99.9% pure CO 2 from the Dashn brewery factory in Gonder, Ethiopia, as the primary feedstock for PCC production.Subsequently, SL was dissolved in distilled water to initiate the dissolution process, leading to the formation of PCC, as outlined in Eq. (1).
In the dissolution step, the SL to distilled water was set at 1:4 (w/v).Precipitation reaction was conducted under atmospheric pressure (1 bar) and at a room temperature of 25 °C.In this process, the Ca(OH) 2 solution was subjected to a reaction with CO 2 , and an excess of CO 2 was introduced into the reactor to ensure complete conversion.The extent of the precipitation reaction was easily monitored by continuously measuring the solution's pH during the precipitation tests.The initial pH of the Ca(OH) 2 solution before the reaction was 13.The pH of (1) CaO (Calcined lime) + H 2 O → Ca (OH) 2 Hydrated lime + Heat the precipitation reaction was measured at 10-min intervals for up to 1 h.Following precipitation, the resulting PCC was filtered and subsequently dried in an oven at a temperature of 120 °C for 24 h.
To facilitate the dissolution process, the SL solution was stirred at a constant temperature of 25 °C and a stirring speed of 150 rpm, varying the mixing time.We measured the pH of the SL solution at 5-min intervals, ranging from 5 to 35 min.Subsequently, we determined the optimal dissolution time when the pH stabilized.The subsequent dissolution step was carried out at this optimized time.After completing the dissolution process, the non-dissolving fraction was removed by sieving through a 100 μm sieve.The pure calcium hydroxide solution was then subjected to a reaction with CO 2 to produce PCC through a precipitation reaction, as depicted in Eq. ( 2).The reaction between the Ca(OH) 2 solution and CO 2 gas took place in the Autoclave.

Characterization
Characterization of both SL and PCC with regard to their crystalline structure was carried out using an X-ray Diffraction (XRD), Minflux 300/600 powder XRD Rigaco (USA).The apparatus was configured with a Cu tube and a CuKα graphic monochromator radiation source, featuring a wavelength (λ) of 1.540593 Å and a scanning speed of 10°/min.The scan ranged from 10 to 70° in 2-Theta, with a step size of 0.02.The recorded results were presented in terms of 2-Theta (degrees) versus intensity (count).For examining the morphology of both SL and PCC fillers, scanning electron microscopy (SEM) was employed as a valuable tool.The morphological characteristics of the samples were analyzed using the FE-SEM/FIB-model Neon-40 (Field emission scanning electron microscope) at the Nanomanufacturing Technology Center (NMTC), CMTI.The particle size distribution was also calculated using the ImageJ version 1.54d 24 by measuring as many pixels as possible.The normality particle size distribution was determined statistically using the Shapiro-Wilk test using RStudio version 2022.07.2 + 576 25 .The chemical oxide compositions of SL and PCC were characterized through a silicate analysis, which included analytical LiBO2 Fusion, HF attack, Gravimetric, Colorimetric, and Atomic absorption spectroscopy (TL-1800AA) [26][27][28][29] .This oxide compositional analysis was conducted at the Ethiopian Geological Survey in Addis Ababa.

PVC compounding
The formulation for manufacturing PVC pipes with a D out of 160 mm and a PN of 10 MPa is detailed in Table 1.This compounding process was carried out at the Amhara pipe factory according to the generalized block flow diagram shown in Fig. 2. To produce a single batch of 160 mm diameter PVC pipes, varying amounts of PCC, ranging from 25 to 100% by weight were combined with a commercial filler.The choice of material handling procedures for creating the primary mix formulations was influenced by the characteristics of the PVC resins  and other additives used in PVC compounds.This research primarily employed the prevalent processing method for PVC compound formulation, known as dry blending or powder mixing, in accordance with ASTM D2396 30 .Dry blending, the industry standard, involves initially processing over 85% of suspension and bulk PVC resins by blending each PVC compounding additive in batch mode using a standard mixer.The formulation included adding 15 PHR of calcium carbonate filler, as indicated in Table 1.The process involved mixing PVC resin powder, stabilizer powder, calcium carbonate powder, titanium dioxide powder, and solid carbon black in a mixer.
A high-temperature mixer, operating at approximately 120 °C, combined these materials at high speed.Once the required temperature was reached, the mixer automatically discharged the mixture into a cooling chamber, reducing the temperature to around 50 °C.Subsequently, pneumatic transportation was used to transfer the mixed materials to a hopper for moisture content removal, followed by transfer to an extruder for melting and achieving homogeneity.The material was then molded using die machines, with the internal diameter adjusted by a Mandrel and the external diameter controlled by the die.The formed pipes were cooled in a vacuum tank equipped with circulating water spray for efficient cooling.Additional cooling was achieved by submerging the pipes in a water bath tank, accompanied by spraying, which was essential to prevent shape deformation resulting from sudden cooling.The hull-off machine provides a constant pulling rate to maintain the proper wall thickness of the finished product followed by cutting according to the required length and finally fitting of the socket at the end of the pipe by using Belling machine.

Mechanical properties of PCC-reinforced PVC
Mechanical properties involved tensile stress at yield, elongation at break, and density measurements.Tensile testing, as per ESISO-6259 31 , involves subjecting a sample to controlled tension until it reaches failure.Parameters directly measured through this test include ultimate tensile strength (stress at yield) and maximum elongation.Specific gravity, which is a measure of density relative to a reference substance (pure water in this case), was determined following ESISO 1183 32 guidelines.If a material has a specific gravity of less than 1, it will float on water.This test method entails weighing a single-piece specimen measured between 1 to 50 g in water, using a sinker made of materials lighter than water.It is suitable for plastics that can become wet but are otherwise unaffected by water.The test specimen consists of a single piece of the material being tested, with no specific size or shape restrictions as long as its volume remains under 1 cm 3 , and its surface and edges are smoothed.The thickness of the specimen for the specific gravity test was adjusted to 1 mm having 1 g of weight.

Effect of pH on the dissolution of the slaked lime and PCC
The dissolution of 1 kg slaked lime sample in 4L water i.e. 1:4 w/v ratios at 25 °C was investigated by varying the dissolution time from 0 to 35 min with increments of 5-min intervals at the fixed mixing rate of 150 rpm.
As can be depicted in Fig. 3, the pH of the dissolved solution was varied with the dissolution time of slaked lime.The pH increased with an increase of dissolution time up to 15 min but remained steady at 13 pH value with further increases of dissolution times.The results depicted that 15 min was enough (optimum) to dissolve the slaked lime powder sample.Therefore, the subsequent precipitation experiments were carried out at this optimum dissolution time (15 min).
The precipitation reaction of the dissolved slaked lime with CO 2 gas in the autoclave was investigated at 1 bar constant pressure and 25 °C temperature for different precipitation times of 10, 20, 30, 40, 50, and 60 min.Figure 3 shows that the pH of the solution decreased as the precipitation time increased.The pH of the solution decreased markedly from 13 to pH 8.6 in 20 min of precipitation after which, the pH decreased slowly until the 40th min, which recorded a pH of ~ 7. The slow decrease in pH was explained by the formation of carbonic acid in the autoclave during precipitation and hence 40 min was selected as the optimum precipitation time for further synthesis and characterization.www.nature.com/scientificreports/

Compositional analysis
The chemical analysis of the SL powder sample is presented in

Thermal analysis
TGA was conducted to assess the thermal stability of the samples at different temperatures.Dynamic thermogravimetric and DTG curves are shown in Fig. 4. The thermogravimetric analysis curve illustrated the weight change of the sample during thermal decomposition over the temperature range of 40-1000 °C.The thermal properties of the PCC are characterized by three main weight loss steps, from 108 to 190 °C, 190 °C to 228 °C, and from 228 to 1000 °C.Corresponding to a 5.48, 20.78 and 72.18 wt.%, respectively.The first phase is attributed to the evaporation of physisorbed water, which are weakly bonded water molecules 33,34 and decarboxylation might occur in the first phase.The second weight loss is attributed to the removal of strongly physisorbed water and was attributed to dehydration and crystallization of amorphous CaCO 3 . 35.The larger weight loss observed was the third phase, which was attributed to the loss of CO 2 from the carbonate decomposition.This high percentage of this phase also provided an independent confirmation that the samples were comprised almost solely of CaCO 3 phases.The DSC curve shows two endothermic phases.The two phases were associated with a broad endothermic peak at 163 °C and a sharp endothermic peak recorded at high temperature (~ 785 °C) which is associated with the melting point of the sample decomposing into CaO 2 .www.nature.com/scientificreports/

Crystallinity analysis
The XRD patterns of PCC demonstrate that there is a slight shift in peak position but the difference is mostly related to the intensity and the sharpness of XRD profiles at the same peak positions.The XRD pattern is presented in Fig. 5.The analysis revealed a difference in structure between raw SL and PCC because the treatment method induce variability in the peak positions and modulation of their intensities.Synthesized PCC has two major intense diffraction peaks at 2θ of 26.58°and 29.54°, which represent Calcite and aragonite, respectively.These peaks appear similarly in SL with values of 26.54° and 29.75° but the intensity is lower than the PCC indicating the formation of concentrated CaCO 3 because of dissolution and precipitation process.PCC also possesses smaller peaks at 2θ of 23.14° which represent the Aragonite type and 2θ at 36.02°, 39.6°, 43.24°, 47.52°, 48.62° and 57.62° in calcite form.Furthermore, the peak at 18.16° in the case of SL was absent in PCC showing the possible development of a crystalline phase.The residual peaks of Ca(OH) 2 noted might be due to the incomplete calcination process.

Surface morphology
The surface morphology of the PCC particles was studied by SEM images as depicted in Fig. 6A-D at different magnifications.Micrographs obtained under different magnifications revealed aggregated mineral flocs (blue circle in Fig. 6A) in the PCC.The images show the existence of both calcite and aragonite (Fig. 6B) crystal structure that possesses different crystal growth patterns.The rod-shaped aragonite crystals are arranged in bundles in a semi-circular structure (white circle in Fig. 6C, D).Similar micrograph observation was reported by previous research 36 .PCC sample appears to be heterogeneous in terms of particle size and shape.It exhibits a narrow size range and rhombohedral crystal shape.The mean particle size of the PCC is found to be 0.05913 μm, which is near the median value of 0.056 indicating the normal distribution of the particle size.The size range of the representative particles is between 0.028 and 0.088 µm.Furthermore, the Shapiro-Wilk test for normality test shows a P value much higher than α (0.05) indicating that the representative particles measured come from normally distributed particles and the normality curve shown in Fig. 7B resembles a Gaussian plot. Figure 7A also indicates the measured particle size follows the Weibull distribution.

Performance analysis of reinforced PVC pipe
Figure 8 shows the filler-mixing ratio against Stress at yield, elongation at break, and density.Overall, the corresponding density values prepared from different filler ratios are within close agreement with each other except for the complete substitution of commercial filler with PCC.As seen from Fig. 8A, the PCC to commercial filler ratio from 25 to 75% resulted in a density within the standard (1.35 to 1.46 g/cm 3 ) recording 1.37 to 1.42 g/cm 3 respectively and 100% substitution resulted in higher density of 1.52 g/cm 3 than the standard.The decrease in density of the composite pipe is attributed to using finer PCC filler indicating that the commercial filler has a smaller particle size than PCC.The use of high-strength fillers is often cost-prohibitive.Here the purity and the particle size of PCC filler significantly affect the mechanical property of the PVC pipe. Figure 8B represents the effect of varying PCC filler ratios on the stress at yield of the developed PVC composites.PCC filler ratio with the commercial filler at 25%, 50%, and 75% is within the standard (> 45 MPa) resulting www.nature.com/scientificreports/ in 47.9, 46.87, and 46.35, respectively.Complete replacement of PCC still results in a slight reduction (44.5 MPa) from the standard as shown by the reference (black line) from Fig. 8C.The decrease in tensile strength at 100% utilization of PCC causes increased brittleness, and this can be attributed to the formation of agglomerates in PCC.These agglomerates form initiation spots of stress concentrations that lead to failure 37,38 .Figure 8C depicted that the PVC sample filled with the filler at all ratios resulted in an elongation at a break above the standard (80%).However, increasing the PCC ratio decreased the elongation at break from 143.64% to 101.35%.This test shows how the commercial filler is more ductile before it breaks.While the synthesized PCC is stiffer, the brittleness increases when increasing the ratio.

Conclusion
Based on the results, the synthesized PCC exhibits significant potential as a filler for compounding PVC, replacing a substantial portion of commercial filler.The pH level emerged as a critical parameter for both dissolution and precipitation processes, with optimal pH values determined to be 13 for dissolved SL and 7.1 for carbonation.Analysis of the synthesized PCC revealed that it primarily consisted of Calcite, with the presence of Aragonite as well, a finding consistent with observations from SEM images.Further examination of chemical analysis indicated that the treated PCC exhibited higher purity (99.2%), aligning with the standard requirements for fillers in the PVC industry.Incorporating PCC alongside commercial CaCO 3 at varying mixing ratios yielded mechanical strength values within acceptable industry limits for stress at yield and density when the PCC content ranged from 25 to 75%.However, complete replacement led to a slight reduction in these properties.Notably, the elongation at break for all mixing ratios conformed to established standards.This research underscores the potential of locally available CaCO 3 as a promising raw material for filler applications.The preliminary experiment signals the possibility of replacing commercial silica with this locally sourced material.

Figure 3 .
Figure 3.Effect of precipitation time on the (A) pH of hydrated SL and (B) PCC solution.

Figure 8 .
Figure 8.Effect of filler ratio on (A) density, (B) Stress at yield, and (C) elongation at break.

Table 1 .
PVC compounding ingredients in PHR.*PCC to commercial filler ratio varied from 25 to 100% by weight.

Table 2 .
The slaked lime powder mainly contains 62.95% CaO and a higher percentage was LOI (27%).The minor oxides presented in small amounts are SiO 2 (4.12%),Al 2 O 3 (1.16%), and MgO (1.05%).SL also contains minor constituents less than 1% i.e.Fe 2 O 3 (0.5%), Na 2 O (0.39%), and K 2 O (0.13%).Chemical analysis of the synthesized PCC sample is also presented in Table2.Chemical analysis of the produced PCC has increased substantially as a result of precipitation reaction to a purity of 99.2% of CaCO 3. The purity of PCC used for PVC pipe formulation should have a purity greater than 98% CaCO 3 that complies with the standard.

Table 2 .
Chemical composition of SL and PCC samples.